1. Field of the Invention
The present invention relates to the field of electronic audio equipment. More particularly, the present invention relates to the field of so-called "noise gates" used in the audio industry to reduce background noise when a signal is absent, or for special effects such as "drum gating" used in music recording studios.
2. Description of the Prior Art
A noise gate can most simply be described as being composed of a threshold detector and a signal controller. The signal controller is used to open the signal path whenever the signal is above a threshold, and close the signal path when the signal falls below the threshold. The signal controller could be in the form of a switch which would cause an abrupt gating of the signal.
More commonly, the signal controller is in the form of a variable gain device producing a ramped gain effect for turn-on and turn-off, allowing a "soft switch" effect or user adjustable specialized gating effect. The turn-on ramp time is normally called the "attack time" of a gate, and the turn-off ramp time is normally called the "release time". A "hold" function is also commonly used which has the effect of keeping the signal controller from immediately beginning to ramp down as soon as the signal goes below threshold. In most conventional noise gates, the threshold, attack time, hold time, and release time are operational adjustments.
A prior art noise gate generally operates as follows. The input signal is applied and feeds to a voltage controlled amplifier (VCA) and a comparator. The comparator also receives an input from a variable reference voltage. When the magnitude of the signal peak is below the magnitude of the reference voltage, the comparator output remains at a logic low level. When a signal peak exceeds the reference magnitude, the comparator output goes to a logic high level. In the latter case, the input signal is said to "go above threshold".
To facilitate proper gate triggering, the signal fed to the comparator is normally a full wave rectified and filtered version of the input signal. The rectification gives an absolute value rendition of the input signal, thus allowing peak detection of both positive and negative signal polarities. The filtering smooths out the rectifier pulses, providing the comparator input which follows the input signal average peak envelope. Without a rectifier filter, a prior art noise gate would undesirably open and close upon each signal wave circle rather than upon the input signal's peak envelope magnitude.
The problem with using a rectifier filter is that it adds an appreciable time delay to the comparator for above and below threshold detection. In addition, it can cause filtering out of fast and short transients of the input signal which may be desirable to retain for gate triggering.
An attack/release switch is used in prior art noise gates, and is controlled by the comparator output. When the input signal is below threshold, the switch remains in a "release" position, and the VCA gain is below unity and the output signal is said to be gated off. When the input signal achieves a great enough magnitude to go above threshold, the comparator output causes the attack/release switch to move to an "attack" position, and the VCA gain is equal to unity and the output signal is now said to be gated on.
In some cases it is desirable to cause a first input signal to be gated on or off by the presence of a second related or unrelated signal. In these cases the comparator receives a separate "triggering" signal which may be independent to the input signal. The first input signal is fed to the VCA and is gated on or off, depending on whether the second triggering signal is above or below the threshold.
Referring to FIG. 1, there is shown the operation of a prior art noise gate. The upper curve shows the VCA Gain (G) and the lower curve shows the signal input amplitude (A) relative to the gating threshold (TH). The horizontal axis of the upper and lower curves are the common time (t) axis. In FIG. 2, it is illustrated that when the input signal rises above threshold TH at time Ta, the VCA gain G begins to ramp up toward 0 dB at the attack rate. As the input signal remains above threshold TH until time Tb, the VCA gain G reaches 0 dB and remains there until time Tb when the signal falls below threshold TH.
The problem with this type of prior art noise gate is that, right after time Tb, even though the input signal still has a significant level just below the gate threshold TH, the VCA begins attenuating along the release time slope between time Tb and time Td (at which time the VCA becomes fully attenuated). This obviously can cause undesired attenuation of an important part of the signal.
To solve this problem, a hold time switch is further utilized in some prior art noise gates. The hold time switch is operated under the control of a hold timer. When the comparator output goes from a high to a low logic level, the hold timer begins a time-out period. During this hold time period, the hold time switch remains in a position which allows the VCA gain to remain unity, and therefore keeps the output signal gated on. This will prevent the quick attenuation of the signal right after the input signal falls below the threshold. When the hold time period lapses, the hold time switch switches to the opposite position which causes the drop of the VCA gain, and therefore causes the output signal to be gated off.
Referring to FIG. 2, there is shown the addition of a significant hold time to the gate. The hold time is initiated at time Tb, when the signal falls below threshold TH, and ends at time Tc. The release time in FIG. 3 is made shorter than that of FIG. 2. It can be seen that the hold time combined with a shorter release time will allow more of the signal to pass through the gate before VCA attenuation begins.
However, even with the hold time function added, the prior art noise gates still have a critical limitation, as illustrated in FIG. 3. Referring to FIG. 3, there is shown that the prior art noise gates cannot function properly when the input signal only rises above the threshold for a brief period of time. As can be seen, the signal now stays above threshold TH for too short a time to allow a full attack to 0 dB VCA gain. Although the hold time is initiated at time Tb when the signal falls below threshold TH, the VCA never attained enough gain to pass the signal at the full level. It is very common in the real world for signals, such as a snare drum being played, to at one time cause the gate to fully open, and at another time to act as shown in FIG. 3, not quite gating fully. This causes difficulty in using the noise gate on such signals.
Therefore, it is very desirable to have a new noise gate device which works properly with all kinds of signals and overcomes the problem associated with prior art noise gate devices.